Winterizing Trees and Shrubs


Sherry Lajeunesse
MSU Extension Urban Pest Management Specialist

BOZEMAN -- Tree and shrub damage caused during winter months often does not show up until spring or early summer, then the problem often gets blamed on other causes.

Preventing the damage is the best course of action, because if your plants begin showing symptoms during the winter there is little you can do except prune the dead wood in the spring.

Winter damage is generally caused by rapid temperature changes or late spring freezes, but seldom by excessive cool alone. Evergreens, such as fir, pine, spruce and juniper are particularly susceptible. Unlike deciduous plants, which lose their leaves in winter, evergreens lose water through the pores in their leaf (needle) surfaces all year round.

Water loss increases when plants are subjected to drying winds. Sudden warm spells when the ground is still frozen, such as the chinooks we receive in parts of Montana, can also be a problem. In a chinook, the needles of evergreens respond to the warmth by opening their pores. This lets water evaporate, but the roots are only be able to draw limited water from the frozen ground.

When the roots cannot keep up with the demand, the needles will turn brown and die. Twigs and buds on the ends become brittle and snap easily when bent.

Any practice that promotes good root growth will help the plant meet its water needs. Watering trees deeply two or three times a year is very helpful. This encourages trees and shrubs to develop deep, strong roots as they reach down into the soil, following the moisture.

Avoid watering trees in late summer or early fall before the leaves fall so they can "harden off" for winter. Then in late fall, after deciduous trees drop their leaves but before the ground freezes, give both evergreen and deciduous trees and shrubs a final deep watering to last them through the winter.

Apply the water under the entire canopy area and beyond. Roots of most deciduous trees extend two-to-three times farther than the branches, roots of evergreens about 1.5 times farther. About 80 to 90 percent of roots of mature trees are found in the upper 18 inches of soil. So allow the water to penetrate about two feet deep. Watering depths can be checked with a soil probe, which is a hollow metal tube that is pushed into the soil or with a soil moisture probe. Agricultural and horticultural supplies often carry these tools.

Covering the ground around shrubs and young trees with a deep mulch can help prevent deep freezing and can help keep the water from being freeze-dried from the soil. This helps keep the water available fro the plants during the winter.

In late fall you can spray foliage of evergreen shrubs and small trees with an anti-transpirant such as Wilt-Pruf of Vapor Guard to cut down on water loss and minimize winter injury. Avoid placing susceptible plants where they will be exposed to wind. Protect established plants by setting up a burlap cloth shelter in later fall.


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